Performance of EUCAST disc diffusion and supplementary methods to detect reduced susceptibility to linezolid in enterococci- the NordicAST LRE-study

OBJECTIVES: This multicentre study aimed to assess performance of the EUCAST disc diffusion (DD) and MIC methods for linezolid susceptibility testing in a genetically diverse strain collection.

METHODS: Nordic clinical microbiology laboratories (n = 83) were invited to test a blinded collection of well-characterized Enterococcus faecalis (n = 10) and Enterococcus faecium (n = 10) strains using the EUCAST DD and gradient tests [Etest and MIC Test strips (MTS)] from two manufacturers (bioMerieux and Liofilchem). Results were compared to reference broth microdilution (BMD) MICs and genotype (presence/absence of linezolid resistance determinants).

RESULTS: Forty-five laboratories provided DD, and 41 gradient test results. Comparing DD with reference MICs and genotype yielded overall categorical agreements (CA) of 87.0% and 97.8%, respectively. Essential agreement (EA) was 99.7% for Etest (bias +10.0%) and 84.5% for MTS (bias +71.0%). Susceptibility categorization with Etest and MTS showed CA of 90.5% and 84.1% with reference MICs, and 82.9% and 98.6% with genotype, respectively. Most discrepancies involved strains with borderline linezolid MICs (4-8 mg/L) and known linezolid resistance mechanisms. Linezolid exposure of strains with MIC 4 mg/L carrying transferable resistance genes (optrA or poxtA), led to MIC increasing above the clinical breakpoint.

CONCLUSIONS: Etest and EUCAST DD had the highest CAs with linezolid reference MIC. EUCAST DD method and MTS gradient test consistently detected strains with confirmed resistance mechanisms. Our findings highlight the potential clinical implications of resistance determinants in MIC borderline strains and support the need for an area of technical uncertainty or a warning in breakpoint tables for linezolid in enterococci.